Archive of Chirp Subbottom Profile, Imagery, and Geospatial Data Collected in 2015 Offshore of Dauphin Island, Alabama

Chirp acquisition - Subbottom data were acquired from the chirp towfish, which was towed by the 54-foot research vessel R/V Irvington and positioned approximately 1 m below the water surface. The seismic source utilized during 2015-330-FA was an EdgeTech SB-512i running DISCOVER version 4.09 acquisition software and towed directly behind the vessel, approximately 19 m behind the GPS reference point. The data were acquired at 6 hertz (Hz), which equates to a roughly 0.2 s sample interval, using a frequency sweep of 0.7-12 kilohertz (kHz), and recording length was 66.47 ms. Based on survey speeds of about 4 knots, the shot spacing is approximately 0.450 m. The binary portion of the seismic data is stored in SEG-Y rev. 0, IBM 32-bit float [big-endian byte order] format, which is a standard digital format that can be read and manipulated by most seismic processing software packages; the first 3,200 bytes of the file header are in ASCII format instead of Extended Binary Coded Decimal Interchange Code (EBCDIC) format. The SEG-Y formatted trace files have a .sgy extension and may be downloaded and processed with commercial or public domain software such as Seismic Unix (SU) (Cohen and Stockwell, 2010). The printable profiles provided are chirp subbottom profile images that were processed using SU software. Survey Field Activity Collection System (FACS) logs and acquisition geometry are also provided as supplemental information in the download file, 2015-330-FA_logs.zip.

Chirp processing - The SEG-Y data were processed with Seismic Unix (Release 44) software to produce gained Graphics Interchange Format (GIF) images of the subbottom profiles included in this data release. A representative chirp data processing sequence consisted of (1) applying a 3000-4000-10,000-12,000 bandpass filter to the analytic data to remove noise from the raw data, prior to creating the envelope seismic data file (corrected files included in this publication have "env_bp" appended to the line name), (2) removing raw [vessel] navigation data for each shot and converting the SEG-Y file to SU format, (3) applying automatic gain control, (4) trimming the trace data TWT to 30 ms (no observable features, aside from multiples, were encountered below that point in the profiles), (5) generating a PostScript image of the traces, and (6) converting the PostScript image to a GIF image.

Navigation acquisition and processing - An Applanix POS MV IANS (positioned amidship) was used for positioning onboard the R/V Irvington. Real-time positions from the POS MV were recorded and written to the seismic trace headers via a RS232 cable running from the POS MV receiver to the topside unit of the chirp system every second. During subbottom processing, location data were extracted from the SEG-Y headers of the processed chirp profiles, using Seismic Unix (Cohen and Stockwell, 2010) software and output as ASCII text files; PROJ.5.1.0 (https://proj.org/) software was used to project WGS84 latitude and longitude coordinates to Universal Transverse Mercator (UTM) northings and eastings (WGS84, Zone 16 North [N], meters). Navigation files for each subbottom line, saved by line number, were concatenated into comma-delimited text files so they could be represented geospatially using Esri ArcGIS software. The final output files are as follows: (1) 2015-330-FA_nav.csv contains unique shotpoint positions (along each line) for the entire survey, (2) 2015-330-FA_1000sht.csv includes the 1,000-shot-interval locations that were generated to correlate shot locations on the trackline map with evenly spaced 1,000-shot index markers along the top of the printable profile images, and (3) 2015-330-FA_sol.csv has the start-of-line locations for each trackline. The concatenated, comma-delimited text files were later imported into ArcMap and saved as either a point or polyline file (in Esri's shapefile format) for use in the map document (.mxd) included in this archive.

Trackline map creation - 2015-330-FA_location.mxd and associated shapefiles (.shp) were created with Esri ArcMap 10.7 software. The survey navigation file, 2015-330-FA_nav.csv, was imported into ArcMap 10.7.1.11595 and processed using the ArcScript "Points to Line" to create a polyline shapefile of the cruise tracklines called 2015-330-FA_trkln. The 1,000-shot-interval and start-of-line files, 2015-330-FA_1000sht.csv and 2015-330-FA_sol.csv, were imported into ArcMap and saved as point shapefiles. A non-proprietary version of the location map was created by utilizing the ArcToolbox "Map to KML" tool, which converted each layer included in the map document (.mxd) to keyhole markup language (.kml) format prior to them being compressed (zipped) and saved as a .kmz file. The ArcGIS and Google Earth geospatial files provided in this publication are unprojected (geographic coordinates, WGS84). For detailed information about the GIS files provided in this data release, please refer to the individual metadata records (*_metadata.txt and *.shp.xml) accompanying each shapefile and the readme file included in 2015-330-FA_gis.zip.